Proficiency Testing Report
NMI RGM 15-03
Liquefied Natural Gas
September 2015
Acknowledgements
This study was conducted by the National Measurement Institute (NMI). Support funding was
provided by the Australian Government Department of Industry & Science.
I would like to thank the management and staff of the participating laboratories for supporting
the study. It is only through widespread participation that we can provide an effective service
to laboratories.
The assistance of the following NMI staff members in the planning, conduct, and reporting of
the study is acknowledged:
John McCallum
Raymond Satumba
---------------------Damian Smeulders
Director Reference Gas Mixtures.
Confidentiality Statement
In this report the participants are identified by a laboratory code that has been assigned by
the NMI. The assigned code is kept confidential.
2
Table of Contents
Acknowledgements.................................................................................................................... 2
Table of Contents ...................................................................................................................... 3
Table of Figures ......................................................................................................................... 4
Summary ................................................................................................................................... 6
1
Introduction ........................................................................................................................ 7
2
Participation ....................................................................................................................... 7
2.1
3
4
5
Laboratory Code ........................................................................................................ 8
Design and Implementation - Study Protocol .................................................................... 8
3.1
Test Mixtures ............................................................................................................. 8
3.2
Analysis and Reporting .............................................................................................. 8
3.3
Schedule .................................................................................................................... 8
Participant Laboratory Information .................................................................................... 9
4.1
Test Method Summaries............................................................................................ 9
4.2
Measurement Uncertainty Estimates......................................................................... 9
4.3
Participant Comments ............................................................................................. 10
Presentation of Results.................................................................................................... 10
5.1
Results Summary .................................................................................................... 10
5.2
Reference Values .................................................................................................... 10
5.3
Traceability .............................................................................................................. 11
5.4
Measurement Uncertainty........................................................................................ 11
5.5
z-Scores ................................................................................................................... 11
5.6
En-Scores ................................................................................................................. 12
6
Proficiency Testing Results ............................................................................................. 13
7
Discussion of Results ...................................................................................................... 58
3
7.1
z-Scores ................................................................................................................... 58
7.2
En-Scores ................................................................................................................ 58
7.3
Measurement of Methane ........................................................................................ 59
7.4
Measurement of Ethane .......................................................................................... 59
7.5
Measurement of Propane ........................................................................................ 59
7.6
Measurement of n-Butane ....................................................................................... 59
7.7
Measurement of iso-Butane .................................................................................... 59
7.8
Measurement of n-Pentane ..................................................................................... 60
7.9
Measurement of iso-Pentane .................................................................................. 60
7.10
Measurement of Oxygen ......................................................................................... 60
7.11
Measurement of Nitrogen ........................................................................................ 60
7.12
Measurement of Carbon Dioxide ............................................................................. 61
8
9
7.13
Measurement Uncertainty........................................................................................ 61
7.14
Calorific Values ........................................................................................................ 62
Reference Values ............................................................................................................ 63
8.1
Reference Values and Traceability.......................................................................... 63
8.2
Assignment and Verification of the Reference Values ............................................ 64
8.3
Measurement Uncertainty of the Reference Values................................................ 64
References ...................................................................................................................... 64
Appendix 1 ............................................................................................................................... 66
Table of Figures
Table 4.1: Test Methods ............................................................................................................ 9
Table 4.2: Measurement Uncertainty Estimates ..................................................................... 10
Table 6.1: Summary of Results. .............................................................................................. 13
Figure 6a: Guide to the Presentation of Results ..................................................................... 19
Figure 6.1: Results for Methane .............................................................................................. 20
Figure 6.2: Results for Ethane ................................................................................................. 21
Figure 6.3: Results for Propane............................................................................................... 22
Figure 6.4: Results for n-Butane.............................................................................................. 23
Figure 6.5: Results for iso-Butane ........................................................................................... 24
Figure 6.6: Results for n-Pentane............................................................................................ 25
Figure 6.7: Results for iso-Pentane ......................................................................................... 26
Figure 6.8: Results for Oxygen ................................................................................................ 27
Figure 6.9: Results for Nitrogen............................................................................................... 28
Figure 6.10: Results for Carbon Dioxide ................................................................................. 29
Figure 6b: Guide to the Presentation of Results – Absolute Difference in Concentration ...... 30
Figure 6.11: Results for Methane ............................................................................................ 31
Figure 6.12: Results for Ethane ............................................................................................... 32
Figure 6.13: Results for Propane............................................................................................. 33
Figure 6.14: Results for n-Butane............................................................................................ 34
Figure 6.15: Results for iso-Butane ......................................................................................... 35
Figure 6.16: Results for n-Pentane.......................................................................................... 36
Figure 6.17: Results for iso-Pentane ....................................................................................... 37
Figure 6.18: Results for Oxygen .............................................................................................. 38
Figure 6.19: Results for Nitrogen............................................................................................. 39
Figure 6.20: Results for Carbon Dioxide ................................................................................. 40
Figure 6c: Guide to the Results of Individual Laboratories...................................................... 41
Figure 6.21: Results for Laboratory A...................................................................................... 42
4
Figure 6.22: Results for Laboratory B...................................................................................... 43
Figure 6.23: Results for Laboratory C ..................................................................................... 44
Figure 6.24: Results for Laboratory D ..................................................................................... 45
Figure 6.25: Results for Laboratory E...................................................................................... 46
Figure 6.26: Results for Laboratory F ...................................................................................... 47
Figure 6.27: Results for Laboratory G ..................................................................................... 48
Table 6.2: z-Scores.................................................................................................................. 49
Table 6.3: En-Scores. .............................................................................................................. 50
Figure 6.28: z-Scores for Methane .......................................................................................... 51
Figure 6.29: z-Scores for Ethane............................................................................................. 51
Figure 6.30: z-Scores for Propane .......................................................................................... 52
Figure 6.31: z-Scores for n-Butane ......................................................................................... 52
Figure 6.32: z-Scores for iso-Butane ....................................................................................... 53
Figure 6.33: z-Scores for n-Pentane ....................................................................................... 53
Figure 6.34: z-Scores for iso-Pentane ..................................................................................... 54
Figure 6.35: z-Scores for Oxygen............................................................................................ 54
Figure 6.36: z-Scores for Nitrogen .......................................................................................... 55
Figure 6.37: z-Scores for Carbon Dioxide ............................................................................... 55
Table 6.4: Calorific Values....................................................................................................... 56
Figure 6.38: Calorific value results .......................................................................................... 57
Figure 8.1: Traceability of the Reference Value ...................................................................... 63
5
Summary
This proficiency testing study was conducted between June 2015 and August 2015 examining
the measurement of the components in liquefied natural gas.
The outcomes of the study were assessed against the aims as follows:
1. To compare the performance of participant laboratories and to assess their
accuracy in the identification and measurement of gas components in a
synthetic natural gas mixture at concentrations that may be typically
encountered in Australian liquefied natural gas (LNG).
Laboratories C and D produced excellent results in this study and for the components that
were measured by these participants, the analytical concentrations agreed with the reference
values to within the stated measurement uncertainties. These two participants produced
satisfactory En-scores for every component that they measured in the gas mixtures.
Laboratories B and E produced very good results in this study with these participants
producing satisfactory z-scores for all components in the mixture. However, the measurement
uncertainties that were reported for some components were insufficient and these produced
unsatisfactory En-scores (Laboratory B: ethane, nitrogen and oxygen. Laboratory E: oxygen).
2. To assess the effect of a range of component concentrations on the
performance of participating laboratories.
Components were present in the gas mixtures ranging in concentration from 88 %mol/mol
(methane) down to 0.007 %mol/mol (oxygen). The spread of concentrations did cause
problems for the participants. There were no issues in the measurement of the high
concentration species methane and ethane. However, the components that were present at
very low concentrations and particularly those that are the main components of air (nitrogen
and oxygen), gave results that were highly variable. Eight out of the nine En-scores calculated
for nitrogen and oxygen were unsatisfactory.
3. To develop the practical application of traceability and measurement
uncertainty and to provide participants with information that will be useful in
assessing their uncertainty estimates.
Chemical testing laboratories accredited to ISO Standard 17025 are required to establish the
traceability and estimate the uncertainty of their test results.
6
•
Reference values for the gas components in these gas mixtures are traceable to the
SI enabling participants to assess the accuracy of test results and the effectiveness of
test methods.
•
Laboratory A did not report measurement uncertainties with their results. Laboratory
C did not report a measurement uncertainty for the measurement of oxygen. All other
participants reported an uncertainty for every measurand.
•
Participants should review their claimed uncertainties to ensure that they are fit for
purpose. The National Measurement Institute can provide advice and training on the
estimation of measurement uncertainty.
1 Introduction
Proficiency testing is an important component of any system of laboratory quality assurance.
Proficiency testing is recognised in ISO/IEC 17025 General Requirements for the
Competence of Testing and Calibration Laboratories1 which lists participation in proficiency
testing programs as an important component of the quality assurance of test results.
The principal aims of the NMI program are:
•
to provide testing laboratories with a tool to improve the accuracy and traceability of
their gas measurements
•
to enable participating laboratories to assess their performance relative to domestic
and international peer laboratories and hence to improve the comparability of results
between laboratories and between countries
In this study, the test samples were prepared and verified by the National Measurement
Institute, then sent to participant laboratories for testing.
Natural gas is a fossil fuel and its economic value per unit of volume or mass is largely
determined by its calorific value. The calorific value and other properties including density are
calculated from compositional data using ISO 6976. Other parameters that might impact the
economic value of natural gas have not been addressed in this PT study.
This PT study was run to support the Australian natural gas industry. The study involved the
determination of the concentration of components in a simulated liquefied natural gas mixture
containing hydrocarbons up to C5. Each laboratory analysed individual gas mixtures
containing the 10 components nitrogen, carbon dioxide, ethane, propane, iso-butane, nbutane, iso-pentane, n-pentane, oxygen and methane at these nominal concentrations:
o
Ethane: 9.7 %
o
Propane: 2 %
o
n-Butane: 0.15 %
o
iso-Butane: 0.15 %
o
n-Pentane: 0.01 %
o
iso-Pentane: 0.02 %
o
Oxygen: 0.01 %
o
Nitrogen: 0.1 %
o
Carbon dioxide: 0.02 %
o
Methane as matrix gas
2 Participation
Six laboratories submitted their results in time to be included in the study report. The
laboratories that participated in the study are listed in Appendix 1.
Participants were permitted to submit multiple sets of results to allow the assessment of
equipment at several sites, or to assess staff competence. One participant submitted two sets
of results (Laboratories F & G) for measurements made at 2 different sites with independent
equipment.
7
2.1 Laboratory Code
To ensure confidentiality, all laboratories were assigned a random code letter on the receipt of
their measurement results.
3 Design and Implementation - Study Protocol
The aims of this study were:
•
To compare the performance of participant laboratories and to assess their accuracy
in the identification and measurement of gas components in a liquefied natural gas
mixture at concentrations that may be encountered.
•
To assess the impact of a range of component concentrations on the performance of
participating laboratories.
•
To develop the practical application of traceability and measurement uncertainty and
provide participants with information that will be useful in assessing their uncertainty
estimates.
3.1 Test Mixtures
Gas mixtures were prepared gravimetrically at the National Measurement Institute, Lindfield.
The composition of the mixtures and their standard uncertainties are shown in Table 6.1.
The preparation of the test mixtures was carried out in accordance with ISO 6142:20014.
After preparation, the compositions of the gas mixtures were verified by comparison with
primary gas standards maintained by the NMI. The mixtures were verified by GC-TCD
(methane, carbon dioxide ethane, oxygen and nitrogen), GC-PDHID (oxygen and nitrogen),
GC-FID (ethane, propane, n-butane, iso-butane, n-pentane and iso-pentane).
3.2 Analysis and Reporting
Participants received one compressed gas cylinder to analyse. Sample cylinders were
dispatched to participants in the week of June 8, 2015. The following items were dispatched
to each participant:
•
One gas cylinder containing the gas sample
•
A form to confirm receipt of the sample
•
A results sheet (an electronic version was supplied by email)
Participating laboratories were requested to specify the methods of measurement used in the
analysis of the gas mixture and each laboratory had to express the uncertainty on all results
submitted as an expanded uncertainty.
Laboratories could nominate the gas components that they wished to measure.
3.3 Schedule
The schedule for this PT Study was as follows.
June 8, 2015
–
Shipment of cylinders to participating laboratories
August 7, 2015
–
Reports of analysis due to the NMI
August 7, 2015
–
Sample cylinders due back at the NMI
September 2015
–
Production of the study report
8
4 Participant Laboratory Information
4.1 Test Method Summaries
Participants were requested to provide a brief summary of their test methods. The test
methods are presented in Table 4.1.
Table 4.1: Test Methods
Lab
Code
Method of Analysis
A
High resolution gas analyzer
B
Agilent 7890 B GC, TCD, Hayesep P 80-100 Mesh.
C
Agilent 7890 B GC, TCD, Wasson-ECE columns
D
Gas chromatography: Alumina Plot 50m column with FID detector, Hayesep
Q and Molsieve column with TCD detector.
E
Shimadzu GC 2014, with TCD & Methaniser FID, MS5a, PPN & Chromosorb
Columns
F-G
The sample was run on two of our three gas chromatographs all Varian 3800
GC TCD/FID with 3 column simultaneous injections (at 2 sites).
V44A Molsieve 5x 45/60, 6’ x 1/8”; Hayesep R 80/100, 3m x 1/8” JW DB1,60m x 0.25mm x 1µm
V44B 13x 45/60, 4’ x 1/8”x2mm, Hayesep P 6’ x 18” CP2062, Chrompak CPSil 5CB 60m x 0.25mm x 1µm
V44C 13x 45/60, 4’ x 1/8”x2mm, Hayesep P 6’ x 18” CP2062, Chrompak CPSil 5CB 60m x 0.25mm x 1µm
4.2 Measurement Uncertainty Estimates
Participants were requested to provide information on the basis of their uncertainty estimates.
The information provided is presented in Table 4.2.
9
Table 4.2: Measurement Uncertainty Estimates
Lab
Code
Uncertainty Estimation Method
A
Not calculated
B
An estimate combining the calibration standard and analysis uncertainties.
C
Site repeatability
D
Relative uncertainty of 2% applied to the result for methane. Relative
uncertainty of 5% applied to the results for ethane, propane, isobutane,
isopentane and pentane.
E
The uncertainty was estimated by combining the uncertainty of the calibration
standards with the uncertainty from the analyses.
F-G
The uncertainty was estimated by combining the uncertainty of the calibration
standards with the uncertainty estimated by statistical analysis from all the
calibration data from all three gas chromatographs since the purchase of the
current gas standards.
4.3 Participant Comments
There were no comments received from the study participants.
5 Presentation of Results
5.1 Results Summary
Measurement results and the estimates of measurement uncertainty reported by participants
are presented in Table 6.1. Graphs of results are presented in Figures 6.1 to 6.20. A guide to
the explanation of these graphs is given in Figure 6a and Figure 6b. In Figures 6.1 to 6.10 all
results are presented as the percentage difference between the reported analytical result and
the reference value assigned by the NMI. Figures 6.11 to 6.20 display the results as the
absolute concentration difference between the reported analytical results and the reference
values.
All measurement uncertainty values are shown as expanded uncertainties at the 95%
confidence level. The expanded uncertainties have been calculated to include the uncertainty
associated with the reference value combined with the uncertainty reported by the participant.
Figures 6.1 to 6.10 have a green line displaying the consensus value which is the average
difference of the participants’ results from the reference value. The consensus value is for
indicative purposes and only the reference values are traceable to the SI.
The results for individual participants are shown in Figures 6.21-6.27. An explanation of the
results for the individual participants is given in Figure 6c.
5.2 Reference Values
The reference values are the best estimate of the true concentration of each component in
the gas mixture. The reference values are gravimetrically determined values that are
traceable to the SI system through the Australian Standard of Mass and through the
Australian national gas standards. All assigned reference values have an accompanying
10
statement of uncertainty that includes contributions from the preparation and verification of
the gas mixtures.
5.3 Traceability
Laboratories accredited to ISO/IEC Standard 170251 must establish and demonstrate the
traceability of their results. Traceability is defined as:
“The property of a measurement result whereby the result can be related to a reference
through a documented unbroken chain of calibrations, each contributing to the measurement
uncertainty.6
The procedure used to establish the traceability of the reference value is described in Section
8.1.
5.4 Measurement Uncertainty
Laboratories accredited to ISO/IEC Standard170251 must estimate the measurement
uncertainty associated with their results. Uncertainty is defined as:
“A parameter, associated with the result of a measurement that characterises the dispersion
of the values that could reasonably be attributed to the measurand.”
The procedures used to establish the uncertainty of the reference values for this study are
described in Section 8.2.
Guidelines for quantifying the uncertainty in analytical measurement are described in
JCGM:1002 and the CITAC/Eurachem Guide3. Information on the analytical uncertainty
reported by participants is presented in Table 4.2.
5.5 z-Scores
z-Scores are a measure of the difference between the reported analytical concentration and
the reference value. Participants’ results were used to calculate z-scores according to the
International Harmonised Protocol5.
z-Scores were calculated using the formula:
z=
χ− X
σ
Where:
z
= z-Score
χ
= Participant result
X
= Reference value
σ
= Target standard deviation
The target standard deviation is a measure of the between-laboratory coefficient of variation
that the study organisers would expect from participants given the concentration of the
components. It is important to note that the target standard deviations (σ) used to calculate zscores are selected by the study coordinator and they are based on practical experience in
the analysis of gas mixtures, the values are not the standard deviations of the participants’
results. In this study the target CV was set at 0.5 % relative for methane, 1 % relative for
ethane and propane; 2 % relative for n-butane and iso-butane; 10% relative for n-pentane,
iso-pentane, carbon dioxide, and nitrogen; and 50% relative for oxygen.
The International Protocol describes how z-scores can be interpreted:
i.
11
An absolute z-score of |z| ≤ 2 indicates a satisfactory result.
ii.
An absolute z-score of 2 < |z| < 3 indicates a questionable result.
iii.
An absolute z-score of |z| ≥ 3 represents an unsatisfactory result.
The z-scores calculated from the participants’ results are shown in Table 6.2 and for the
individual components in Figures 6.28-6.37.
5.6 En-Scores
Although z-scores are a useful indicator of laboratory performance, they do not take into
account the uncertainties associated with reported results and reference values. Without an
assessment of the uncertainty, it is not normally possible to judge the fitness for purpose of
the test result. En-scores do take measurement uncertainty into account and are
complementary to z-scores in the assessment of laboratory performance.
i.
An absolute En-score of ≤ |1| indicates a satisfactory result. The reported result and
reference value are in agreement (within their respective uncertainties).
ii.
An absolute En-score of > |1| indicates an unsatisfactory result. The reported result is
different to the reference value and the uncertainty associated with the result has
been understated.
If the uncertainty reported with the result is large enough, the absolute En-score will always be
<1. However, by examining both z-scores and En-scores together a judgement can be made
about the accuracy and fitness for purpose of the test result. The En-scores calculated from
the participants’ results are shown in Table 6.3.
12
6 Proficiency Testing Results
Table 6.1: Summary of Results.
In this table the results from the proficiency testing study are summarised. The following data is presented:
xprep
amount of substance ratio, from preparation (Reference Value)
uprep
standard uncertainty from the manufacture of the sample
uverify
standard uncertainty from the verification testing of the sample
uref
standard uncertainty of the Reference Value
xlab
result from participant laboratory
Ulab
stated expanded uncertainty from participant laboratory, at 95% level of confidence
klab
stated coverage factor
Δx
difference between laboratory result and Reference Value
Δx/x
relative difference between laboratory result and Reference Value (as a percentage)
k
assigned coverage factor
U(Δx)
expanded uncertainty of difference Δx, at 95% level of confidence. Calculated using the equation:
U(Δx) = k .[( uref ) 2+ ( ulab ) 2] ½
U(Δx)/x relative expanded uncertainty of difference Δx, at 95% level of confidence (as a percentage)
Key: NT = Not Tested
13
Table 6.1: Summary of Results.
Gas component: Methane
Code
x prep
u prep
u verify
%mol/mol
u ref
x lab
U lab
%mol/mol
%mol/mol
%mol/mol
k lab
Δx
Δx/x
k
U(Δx)
U(Δx)/x
-0.02
0.0%
2
0.03
0.0%
A
87.417
0.001
0.015
0.015
87.4
B
88.053
0.001
0.015
0.015
88.072
0.035
2
0.019
0.0%
2
0.046
0.1%
C
87.803
0.001
0.015
0.015
87.82
0.0896
2
0.02
0.0%
2
0.094
0.1%
D
87.417
0.001
0.015
0.015
88.2
1.8
2
0.8
0.9%
2
1.8
2.1%
E
88.053
0.001
0.015
0.015
88.0
0.5
2
-0.1
-0.1%
2
0.5
0.6%
F
87.417
0.001
0.015
0.015
87.312
1.012
1.96
-0.105
-0.1%
2
1.033
1.2%
G
87.417
0.001
0.015
0.015
87.263
1.012
1.96
-0.154
-0.2%
2
1.033
1.2%
Δx
Δx/x
k
U(Δx)
U(Δx)/x
-0.2
-1.8%
2
0.006
0.1%
(Table 6.1 continued) Gas component: Ethane
Code
x prep
u prep
u verify
%mol/mol
14
u ref
x lab
U lab
%mol/mol
%mol/mol
%mol/mol
k lab
A
10.082
0.001
0.003
0.003
9.9
B
9.814
0.001
0.003
0.003
9.803
0.007
2
-0.011
-0.1%
2
0.009
0.1%
C
9.538
0.001
0.003
0.003
9.53
0.0445
2
-0.01
-0.1%
2
0.045
0.5%
D
10.082
0.001
0.003
0.003
10.2
0.5
2
0.1
1.2%
2
0.5
5.0%
E
9.814
0.001
0.003
0.003
9.86
0.09
2
0.05
0.5%
2
0.09
0.9%
F
10.082
0.001
0.003
0.003
10.155
0.448
1.96
0.073
0.7%
2
0.457
4.5%
G
10.082
0.001
0.003
0.003
10.259
0.448
1.96
0.177
1.8%
2
0.457
4.5%
(Table 6.1 continued) Gas component: Propane
Code
x prep
u prep
u verify
%mol/mol
u ref
x lab
U lab
%mol/mol
%mol/mol
%mol/mol
k lab
Δx
Δx/x
k
U(Δx)
U(Δx)/x
-0.1
-7.0%
2
0.004
0.2%
A
2.042
0.0007
0.0019
0.002
1.9
B
1.654
0.0007
0.0019
0.002
1.650
0.004
2
-0.004
-0.2%
2
0.006
0.3%
C
2.157
0.0007
0.0019
0.002
2.16
0.0255
2
0.00
0.2%
2
0.026
1.2%
D
2.042
0.0007
0.0019
0.002
2.07
0.10
2
0.03
1.4%
2
0.10
4.9%
E
1.654
0.0007
0.0019
0.002
1.66
0.04
2
0.01
0.4%
2
0.04
2.4%
F
2.042
0.0007
0.0019
0.002
2.039
0.440
1.96
-0.003
-0.2%
2
0.449
22.0%
G
2.042
0.0007
0.0019
0.002
2.009
0.440
1.96
-0.033
-1.6%
2
0.449
22.0%
u ref
x lab
U lab
k lab
Δx
Δx/x
k
U(Δx)
U(Δx)/x
%mol/mol
%mol/mol
%mol/mol
-0.02
-15.9%
2
0.0004
0.3%
(Table 6.1 continued) Gas component: n-Butane
Code
x prep
u prep
u verify
%mol/mol
15
A
0.1189
0.0001
0.0002
0.0002
0.1
B
0.1309
0.0001
0.0002
0.0002
0.1310
0.0005
2
0.0001
0.1%
2
0.0006
0.5%
C
0.1630
0.0001
0.0002
0.0002
0.16
0.0088
2
0.00
-1.8%
2
0.0088
5.4%
D
0.1189
0.0001
0.0002
0.0002
0.120
0.006
2
0.001
0.9%
2
0.006
5.1%
E
0.1309
0.0001
0.0002
0.0002
0.129
0.013
2
-0.002
-1.5%
2
0.013
9.9%
F
0.1189
0.0001
0.0002
0.0002
0.116
0.045
1.96
-0.003
-2.5%
2
0.046
38.6%
G
0.1189
0.0001
0.0002
0.0002
0.111
0.045
1.96
-0.008
-6.7%
2
0.046
38.6%
(Table 6.1 continued) Gas component: iso-Butane
Code
x prep
u prep
u verify
%mol/mol
u ref
x lab
U lab
%mol/mol
%mol/mol
%mol/mol
k lab
Δx
Δx/x
k
U(Δx)
U(Δx)/x
-0.05
-35.1%
2
0.0005
0.4%
A
0.1542
0.0001
0.00026
0.0003
0.1
B
0.1653
0.0001
0.00026
0.0003
0.1655
0.0005
2
0.0002
0.1%
2
0.0007
0.4%
C
0.1542
0.0001
0.00026
0.0003
0.15
0.0046
2
0.00
-2.7%
2
0.0046
3.0%
D
0.1542
0.0001
0.00026
0.0003
0.156
0.008
2
0.002
1.2%
2
0.008
5.2%
E
0.1653
0.0001
0.00026
0.0003
0.162
0.016
2
-0.003
-2.0%
2
0.016
9.7%
F
0.1542
0.0001
0.00026
0.0003
0.152
0.058
1.96
-0.002
-1.4%
2
0.059
38.4%
G
0.1542
0.0001
0.00026
0.0003
0.147
0.058
1.96
-0.007
-4.6%
2
0.059
38.4%
u ref
x lab
U lab
k lab
Δx
Δx/x
k
U(Δx)
U(Δx)/x
%mol/mol
%mol/mol
%mol/mol
(Table 6.1 continued) Gas component: n-Pentane
Code
x prep
u prep
u verify
%mol/mol
16
A
0.0100
0.00002
0.00012
0.0001
<0.1
B
0.0092
0.00002
0.00012
0.0001
0.0093
0.0002
2
0.0001
1.5%
2
0.0003
3.5%
C
0.0101
0.00002
0.00012
0.0001
0.01
0.0037
2
0.00
-1.2%
2
0.0037
36.7%
D
0.0100
0.00002
0.00012
0.0001
0.0100
0.0005
2
0.0000
0.3%
2
0.0006
5.6%
E
0.0092
0.00002
0.00012
0.0001
0.0095
0.0010
2
0.0003
3.7%
2
0.0010
11.2%
F
0.0100
0.00002
0.00012
0.0001
0.008
0.005
1.96
-0.002
-19.8%
2
0.005
51.2%
G
0.0100
0.00002
0.00012
0.0001
0.009
0.005
1.96
-0.001
-9.7%
2
0.005
51.2%
(Table 6.1 continued) Gas component: iso-Pentane
Code
x prep
u prep
u verify
%mol/mol
u ref
x lab
U lab
%mol/mol
%mol/mol
%mol/mol
k lab
Δx
Δx/x
k
U(Δx)
U(Δx)/x
A
0.0223
0.00003
0.00009
0.0001
<0.1
B
0.0235
0.00003
0.00009
0.0001
0.0235
0.0002
2
0.0000
0.1%
2
0.0003
1.2%
C
0.0209
0.00003
0.00009
0.0001
0.02
0.0063
2
-0.001
-4.4%
2
0.0063
30.1%
D
0.0223
0.00003
0.00009
0.0001
0.0228
0.0011
2
0.0005
2.2%
2
0.0011
5.0%
E
0.0235
0.00003
0.00009
0.0001
0.0250
0.0025
2
0.0015
6.5%
2
0.0025
10.7%
F
0.0223
0.00003
0.00009
0.0001
0.022
0.012
1.96
0.000
-1.3%
2
0.012
54.9%
G
0.0223
0.00003
0.00009
0.0001
0.021
0.012
1.96
-0.001
-5.8%
2
0.012
54.9%
u ref
x lab
U lab
k lab
Δx
Δx/x
k
U(Δx)
U(Δx)/x
%mol/mol
%mol/mol
%mol/mol
0.09
1406.0%
2
0.0004
6.0%
-0.0013
-19.9%
2
0.0004
6.2%
0.00
15.1%
2
0.0004
4.6%
(Table 6.1 continued) Gas component: Oxygen
Code
x prep
u prep
u verify
%mol/mol
A
0.0066
0.00001
0.0002
0.0002
0.1
B
0.0065
0.00001
0.0002
0.0002
0.00520
C
0.0087
0.00001
0.0002
0.0002
0.01
D
17
0.00003
2
NT
E
0.0065
0.00001
0.0002
0.0002
0.0050
0.0005
2
-0.0015
-23.0%
2
0.0006
9.9%
F
0.0066
0.00001
0.0002
0.0002
0.016
0.002
1.96
0.009
141.0%
2
0.002
31.3%
G
0.0066
0.00001
0.0002
0.0002
0.011
0.002
1.96
0.004
65.7%
2
0.002
31.3%
(Table 6.1 continued) Gas component: Nitrogen
Code
x prep
u prep
u verify
%mol/mol
u ref
x lab
U lab
%mol/mol
%mol/mol
%mol/mol
k lab
Δx
Δx/x
k
U(Δx)
U(Δx)/x
0.27
199.6%
2
0.0008
0.6%
A
0.1335
0.0001
0.0004
0.0004
0.4
B
0.1305
0.0001
0.0004
0.0004
0.1273
0.001
2
-0.0032
-2.4%
2
0.001
1.0%
C
0.1319
0.0001
0.0004
0.0004
0.13
0.0038
2
0.00
-1.4%
2
0.0039
2.9%
D
NT
E
0.1305
0.0001
0.0004
0.0004
0.110
0.007
2
-0.020
-15.7%
2
0.007
5.4%
F
0.1335
0.0001
0.0004
0.0004
0.167
0.021
1.96
0.033
25.1%
2
0.021
16.1%
G
0.1335
0.0001
0.0004
0.0004
0.157
0.021
1.96
0.023
17.6%
2
0.021
16.1%
u ref
x lab
U lab
k lab
Δx
Δx/x
k
U(Δx)
U(Δx)/x
%mol/mol
%mol/mol
%mol/mol
(Table 6.1 continued) Gas component: Carbon Dioxide
Code
x prep
u prep
u verify
%mol/mol
A
0.0136
0.00001
0.0001
0.0001
<0.1
B
0.0133
0.00001
0.0001
0.0001
0.0138
0.0005
2
0.0005
4.0%
2
0.0006
4.3%
C
0.0134
0.00001
0.0001
0.0001
0.01
0.0172
2
0.00
-25.5%
2
0.0172
128.2%
D
18
NT
E
0.0133
0.00001
0.0001
0.0001
0.0130
0.0007
2
0.000
-2.1%
2
0.0008
5.7%
F
0.0136
0.00001
0.0001
0.0001
0.013
0.001
1.96
-0.001
-4.3%
2
0.001
7.8%
G
0.0136
0.00001
0.0001
0.0001
0.012
0.001
1.96
-0.002
-11.6%
2
0.001
7.8%
Difference from reference value (% relative)
20%
15%
Difference between the
concentration determined
by the laboratory and the
gravimetric reference
value (as a % relative
value)
10%
5%
The expanded uncertainty includes the uncertainty
of the reference value and the uncertainty reported
by the participating laboratory. The expanded
uncertainty is shown as a % relative value.
0%
-5%
A value close to zero,
indicates that the
laboratory determined
concentration agrees
with the reference value
-10%
-15%
A
B
C
D
The green line is the
consensus value
calculated from the
average difference from
the reference value
E
Laboratory
Figure 6a: Guide to the Presentation of Results
19
F
G
H
I
Figure 6.1: Results for Methane
20
Figure 6.2: Results for Ethane
21
Figure 6.3: Results for Propane
22
Figure 6.4: Results for n-Butane
23
-35.1%
Figure 6.5: Results for iso-Butane
24
Figure 6.6: Results for n-Pentane
25
Figure 6.7: Results for iso-Pentane
26
1400%
Figure 6.8: Results for Oxygen
27
200%
Figure 6.9: Results for Nitrogen
28
Figure 6.10: Results for Carbon Dioxide
29
Absolute difference from reference value
(%mol/mol)
0.5
0.4
Difference between the
concentration determined
by the laboratory and the
gravimetric reference
value (as an absolute
value)
0.3
0.2
The green line is the
consensus value calculated
from the average difference
from the reference value
0.1
0.0
-0.1
-0.2
-0.3
The expanded uncertainty includes the uncertainty of
the reference value and the uncertainty reported by the
participating laboratory. The expanded uncertainty is
shown as an absolute value.
A value close to zero,
indicates that the
laboratory determined
concentration agrees
with the reference value
Table 6.1 shows the equation used to calculate this
value
-0.4
A
B
C
D
E
F
Laboratory
Figure 6b: Guide to the Presentation of Results – Absolute Difference in Concentration
30
G
H
I
Figure 6.11: Results for Methane
31
Figure 6.12: Results for Ethane
32
Figure 6.13: Results for Propane
33
Figure 6.14: Results for n-Butane
34
Figure 6.15: Results for iso-Butane
35
Figure 6.16: Results for n-Pentane
36
Figure 6.17: Results for iso-Pentane
37
0.09
Figure 6.18: Results for Oxygen
38
0.27
Figure 6.19: Results for Nitrogen
39
Figure 6.20: Results for Carbon Dioxide
40
Difference between the concentration
determined by the laboratory and the
gravimetric reference value (as a %
relative value)
Identity of gas
component
Expanded uncertainty:
including the uncertainty
reported by the participant
and the uncertainty of the
reference value
Figure 6c: Guide to the Results of Individual Laboratories.
41
Relative difference for the
calorific value calculated
from the reported
measurement results
Figure 6.21: Results for Laboratory A
42
Oxygen
Nitrogen
1406%
200%
Figure 6.22: Results for Laboratory B
43
Figure 6.23: Results for Laboratory C
44
Figure 6.24: Results for Laboratory D
45
Figure 6.25: Results for Laboratory E
46
Oxygen 141%
Figure 6.26: Results for Laboratory F
47
Oxygen 66%
Figure 6.27: Results for Laboratory G
48
Table 6.2: z-Scores.
z-Scores are a measure of the difference between the reported result and the reference value.
•
An absolute z-score of |z| ≤ 2 indicates a satisfactory result.
•
An absolute z-score of 2 < |z| < 3 indicates a questionable result.
•
An absolute z-score of |z| ≥ 3 represents an unsatisfactory result
Code
49
Methane
Ethane
Propane
n-Butane
iso-Butane
n-Pentane
iso-Pentane
Oxygen
Carbon
Dioxide
Nitrogen
A
0.0
-1.8
-7.0
-8.0
-17.6
B
0.0
-0.1
-0.2
0.0
0.1
0.2
0.0
-0.4
0.4
-0.2
C
0.0
-0.1
0.2
-0.9
-1.4
-0.1
-0.4
0.3
-2.5
-0.1
D
1.8
1.2
1.4
0.4
0.6
0.0
0.2
E
-0.1
0.5
0.4
-0.7
-1.0
0.4
0.7
-0.5
-0.2
-1.6
F
-0.2
0.7
-0.2
-1.2
-0.7
-2.0
-0.1
2.8
-0.4
2.5
G
-0.4
1.8
-1.6
-3.3
-2.3
-1.0
-0.6
1.3
-1.2
1.8
28.1
20.0
Table 6.3: En-Scores.
En-Scores are a measure of the agreement between the reported result and the reference value
•
An absolute En-score of ≤ |1| indicates a satisfactory result. The reported result and reference value are in agreement.
•
An absolute En-score of > |1| indicates an unsatisfactory result.
Code
Methane
Ethane
Propane
n-Butane
iso-Butane
n-Pentane
iso-Pentane
Oxygen
Carbon
Dioxide
-3.2
0.9
-2.5
-0.2
-0.5
Nitrogen
A
50
B
0.4
-1.2
-0.6
0.1
0.2
0.4
0.1
C
0.2
-0.2
0.1
-0.3
-0.9
0.0
-0.1
D
0.4
0.2
0.3
0.2
0.2
0.1
0.4
E
-0.1
0.5
0.2
-0.1
-0.2
0.3
0.6
-2.3
-0.4
-2.9
F
-0.1
0.2
0.0
-0.1
0.0
-0.4
0.0
4.6
-0.6
1.6
G
-0.2
0.4
-0.1
-0.2
-0.1
-0.2
-0.1
2.1
-1.5
1.1
Figure 6.28: z-Scores for Methane
Figure 6.29: z-Scores for Ethane
51
Figure 6.30: z-Scores for Propane
Figure 6.31: z-Scores for n-Butane
52
Figure 6.32: z-Scores for iso-Butane
Figure 6.33: z-Scores for n-Pentane
53
Figure 6.34: z-Scores for iso-Pentane
Figure 6.35: z-Scores for Oxygen
54
Figure 6.36: z-Scores for Nitrogen
Figure 6.37: z-Scores for Carbon Dioxide
55
Table 6.4: Calorific Values.
In this table the results from the PT study have been converted into the calorific value (energy content) of the natural gas. The superior calorific values have
been calculated using ISO 6976, Tables 3 & 5 at the standard conditions 15 ºC /15ºC in accordance with AS ISO 13443. The following data is presented:
H ref
The calorific value of the sample calculated from the reference values
H lab
The calorific value calculated from the measurement results for each participant
u(H ref)
The standard uncertainty of the calorific value calculated from the reference values
u(H lab)
The standard uncertainty of the calorific value calculated from the measurement results
∆H
The relative difference between the calorific values calculated from the reference values and from the measurement results.
U(H)
The combined expanded uncertainty (as a % relative value)
Reference Value
Difference
Uncertainty
H ref
u(H ref)
H lab
u(H lab)
(∆H)
U(H)
MJ/m3
MJ/m3
MJ/m3
MJ/m3
%
%
A
41.95
0.03
41.59
0.00
-0.87%
0.13%
B
41.68
0.03
41.68
0.01
-0.01%
0.13%
C
42.00
0.03
41.90
0.03
-0.24%
0.18%
D
41.95
0.03
42.36
0.38
0.96%
1.82%
E
41.68
0.03
41.69
0.10
0.02%
0.50%
F
41.95
0.03
41.95
0.32
-0.01%
1.54%
G
41.95
0.03
41.96
0.32
0.02%
1.54%
Code
56
As Analysed
Figure 6.38: Calorific value results
57
7 Discussion of Results
Laboratories C and D produced excellent results in this study. For all measured components,
these laboratories reported measurement results that closely agreed with the reference
values to within the stated measurement uncertainties.
Laboratories B and E produced very good results and obtained satisfactory z-scores for all
measured components. However, these laboratories reported measurement uncertainties that
produced unsatisfactory En-Scores for some components (Laboratory B: ethane, nitrogen and
oxygen. Laboratory E: Oxygen).
Each laboratory that produced an unsatisfactory result should examine their measurement
uncertainty values and make a decision on the fitness for purpose based on the analyses that
they typically perform. Laboratories should perform a corrective action for each result that
produced an unacceptable z-score or En-score.
7.1 z-Scores
z-Scores achieved by the study participants are shown in Table 6.2 and in Figures 6.28 6.37. Of the 64 analysis results reported, 54 results (84 %) produced an absolute z-score ≤ 2,
indicating satisfactory results.
•
Laboratories B, D and E produced satisfactory z-scores for all measured
components.
•
Laboratory C produced a questionable z-score for one of the measured components
(carbon dioxide).
•
All other study participants obtained more than one questionable or unsatisfactory zscore.
7.2 En-Scores
En-scores achieved by the study participants are shown in Table 6.3. Of the 56 En-scores
calculated, 46 results (82 %) returned an absolute En-score ≤ 1, indicating that the reported
results were in agreement with the reference values to within the claimed measurement
uncertainties. 80% of the unsatisfactory En-scores were obtained for nitrogen and oxygen.
These two components were present at very low concentrations in the PT samples, and the
measurement of these species is susceptible to interference from the presence of air during
the analysis of the sample or standards.
58
•
Laboratories C and D returned absolute En-scores ≤ 1 for each measured component.
•
Laboratory B reported excellent results that agreed closely with the reference values.
However, the very small measurement uncertainties reported by this participant
produced unsatisfactory En-Scores for three of the components (ethane, nitrogen and
oxygen). The small negative bias in the measurement of oxygen and nitrogen
suggests that the standard used by the laboratory has been contaminated with a
small amount of air.
•
Laboratory A did not report measurement uncertainties. En-scores have not been
calculated for this participant.
•
All other study participants obtained more than one unsatisfactory En-score.
7.3 Measurement of Methane
Methane was the matrix gas and was present at high concentrations in the gas mixtures at
close to 88 % of the total mole fraction. Every participant measured the methane content with
all results producing satisfactory z-scores. In addition, all measurement results for methane
agreed with the reference values to within the claimed uncertainties.
The results for methane are shown in Figures 6.1 and 6.11.
7.4 Measurement of Ethane
Ethane was present in the gas mixtures at concentrations close to 10 % of the total mole
fraction.
•
Every participant measured ethane and all measurement results produced
satisfactory z-scores.
•
Laboratory B obtained an unsatisfactory En-score for the measurement of ethane
indicating that the measurement result did not agree with the reference value to within
the claimed measurement uncertainty. Laboratory B claimed a very small uncertainty
for this component.
The results for ethane are shown in Figures 6.2 and 6.12.
7.5 Measurement of Propane
Propane was present in the gas mixtures at concentrations close to 2 % of the total mole
fraction.
•
Laboratory A obtained an unsatisfactory z-score for this component. This laboratory
used a calibration standard with a very different composition to the sample being
analysed - possibly introducing biases in the testing of the sample.
•
All other participants obtained satisfactory z-scores.
•
Laboratories B, C, D, E, F and G obtained En-scores ≤ 1 indicating that the
measurement results agreed with the reference values to within the claimed
measurement uncertainties.
The results for propane are shown in Figures 6.3 and 6.13.
7.6 Measurement of n-Butane
n-Butane was present in the LNG mixtures at concentrations between 0.11 – 0.17 % of the
total mole fraction.
•
Laboratories A and G obtained unsatisfactory z-scores for this component. All other
participants obtained satisfactory z-scores.
•
Laboratories B, C, D, E, F and G obtained satisfactory En-scores for the
measurement of this component.
The results for n-butane are shown in Figures 6.4 and 6.14.
7.7 Measurement of iso-Butane
iso-Butane was present in the LNG mixtures at concentrations close to 0.15 % of the total
mole fraction.
59
•
Laboratory A obtained an unsatisfactory z-score for this component.
•
Laboratory G obtained a questionable z-score for this component.
•
Laboratories B, C, D, E, F and G obtained satisfactory En-scores for the
measurement of this component.
•
The laboratory G measurements were made on a subsample that was decanted from
the sample analysed at laboratory F. The results for n-butane and iso-butane could
indicate that the sample was altered during the decanting process.
The results for iso-butane are shown in Figures 6.5 and 6.15.
7.8 Measurement of n-Pentane
n-Pentane was present at very low concentrations close to 0.01 % of the total mole fraction.
•
Laboratory A reported a value of <0.1 % for this component. n-Pentane was present
at a concentration below the limit of detection for the laboratory’s analysis equipment.
•
Laboratories B, C, D, E, F and G obtained satisfactory z-scores and En-scores for the
measurement of this component.
The results for n-pentane are shown in Figures 6.6 and 6.16.
7.9 Measurement of iso-Pentane
iso-Pentane was present at concentrations close to 0.02 % of the total mole fraction.
•
Laboratory A reported a value of <0.1 % for this component. iso-Pentane was present
at a concentration below the limit of detection for the laboratory’s analysis equipment.
•
Laboratories B, C, D, E, F and G obtained satisfactory z-scores and En-scores for the
measurement of this component.
The results for iso-pentane are shown in Figures 6.7 and 6.17.
7.10 Measurement of Oxygen
Oxygen was present at concentrations close to 0.007 % of the total mole fraction. Oxygen
was a very minor component but was added to allow the estimation of air contamination
during testing. Oxygen is a difficult species to quantify, particularly when it is present at such
low concentrations. Contamination of the sample cylinder or the calibration standards during
the connection and purging of regulators, or the presence of air leaks in the gas sampling
system, will lead to an incorrect measurement of the oxygen content when it is present at
such low levels.
•
Laboratory D did not measure the oxygen content.
•
Laboratory A obtained an unsatisfactory z-score for this component.
•
Laboratory F obtained a questionable z-score for this component.
•
Laboratory C did not estimate the measurement uncertainty for their measurement of
oxygen. An En-score has not been calculated for this measurement.
•
Laboratories B, E, F and G underestimated the measurement uncertainty for the
analysis of oxygen and each of these participants obtained an unsatisfactory Enscore.
The results for oxygen are shown in Figures 6.8 and 6.18.
7.11 Measurement of Nitrogen
Nitrogen was present in the study mixtures at concentrations close to 0.13 % of the total mole
fraction.
60
•
Laboratory D did not measure the nitrogen content.
•
Laboratory A obtained an unsatisfactory z-score for this component.
•
Laboratory F obtained a questionable z-score for this component.
•
Laboratories B, E, F and G underestimated the measurement uncertainty for the
analysis of nitrogen oxygen and each of these participants obtained an unsatisfactory
En-score.
The results for nitrogen are shown in Figures 6.9 and 6.19.
Laboratories B, E, F and G all produced measurement results for both oxygen and nitrogen
that did not agree with the reference values, and in all cases the results for both components
were biased in the same direction relative to the reference values. Laboratories B and E
reported results that were biased low – suggesting a small contamination of the calibration
standard with air. For laboratory B this contamination was negligible and represented less
than 50 ppm of air introduced into the calibration standard. Laboratories F and G reported
results that were biased high – suggesting a small contamination of the PT sample with air.
7.12 Measurement of Carbon Dioxide
Carbon dioxide was present in the study mixtures at concentrations close to 0.013 % of the
total mole fraction.
•
Laboratory A reported a value of <0.1 % for this component. Carbon dioxide was
present at a concentration below the limit of detection for the laboratory’s analysis
equipment.
•
Laboratory D did not measure the carbon dioxide content.
•
Laboratory C obtained a questionable z-score for this component.
•
Laboratory G obtained an unsatisfactory En-score for this component.
•
Laboratories B, E and F obtained satisfactory z-scores and En-scores for the
measurement of carbon dioxide.
The results for carbon dioxide are shown in Figures 6.10 and 6.20.
7.13 Measurement Uncertainty
Participants were requested to report the basis of their measurement uncertainty estimates
and this information is presented in Table 4.2. According to ISO/IEC Standard 17025, there is
a requirement for laboratories to report an estimation of the measurement uncertainty. Clause
5.4.6.2 of the Standard states:
“Testing laboratories shall have and shall apply procedures for estimating uncertainty of
measurement. In certain cases the nature of the test method may preclude rigorous,
metrologically and statistically valid calculation of uncertainty of measurement. In these areas
the laboratory should at least attempt to identify all the components of uncertainty and make a
reasonable estimation, and shall ensure that the form of reporting of the result does not give a
wrong impression of the uncertainty. Reasonable estimation shall be based on knowledge of
the performance of the method and on the measurement scope and shall make use of, for
example previous experience and validation data”.
The measurement uncertainty estimates that were provided by participants are shown in
Table 6.1.
•
61
Laboratory A did not report measurement uncertainties for their results.
•
Laboratory C did not report an uncertainty for the measurement of oxygen.
The measurement uncertainties reported by all other participants have been converted into
relative uncertainties and are shown as error bars in Figures 6.1 - 6.10 and Figures 6.21 6.27. The measurement uncertainties in Figures 6.11 - 6.20 have been calculated as absolute
values. The high proportion of results that produced En-scores <1 demonstrated that the
participants have a good understanding of the sources of uncertainty in this type of testing.
Laboratories that obtained unsatisfactory En-scores should review their measurement
uncertainty calculations to ensure that all sources of uncertainty have been included in their
combined uncertainty estimates. The National Measurement Institute runs regular training
courses on the estimation of measurement uncertainty.
7.14 Calorific Values
Table 6.4 displays the calorific values calculated from participant’s results; this table also
shows the difference in the calorific value calculated from a participant’s results against the
true calorific value of the gas mixture.
The calorific values were calculated using ISO 6976 8 at the reference conditions detailed in
AS ISO 13443 9.
•
Measurement uncertainty estimates have been calculated for the calorific values in
Table 6.4. The combined expanded uncertainties U(H) include contributions from the
uncertainty of the reference values, the uncertainty of the measurement results, and a
standard uncertainty (0.07 % relative) that was applied to the values in Table 5 of ISO
6976.
The calorific values calculated for each participant are displayed in Figure 6.38 and in the
individual results for each laboratory (Figures 6.21-6.27).
For companies that export natural gas, the energy content must be specified to within 0.1 %
of the true calorific value. Analysing LNG to obtain a calorific value at that level of accuracy is
a significant measurement challenge. Figure 6.38 displays the 0.1 % accuracy target for the
calorific value as red dotted lines. In this study, laboratories B, E, F and G reported
measurement results that produced calorific values that meet international requirements.
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8 Reference Values
8.1 Reference Values and Traceability
In the metrology of chemistry there are two aspects of traceability – the identity of the analyte
and the amount of substance. The identity of the analyte has been established by the
manufacturer of the pure gases used in the production of these gas mixtures.
The amount of substance is determined by mass, through a system of calibrated masses and
balances that are directly traceable to the SI and by the purity of the gas standards as shown
below in Figure 8.1. Traceability to the SI could have also been attained by analytical testing
through the use of a validated test method and the use of gas standards traceable to the mole
via primary standards.
Figure 8.1: Traceability of the Reference Value
AMOUNT OF SUBSTANCE
MOLE1
MASS KILOGRAM1
International prototype kilogram2
Australian standard of mass3
Calibrated masses at NMI
Source Gases
Method validation
•
Identity
•
Chemical Identity
•
Impurities
•
Quality Control
•
Molecular weight
•
Bias Control
Calibrated balances at NMI
Mass of pure gases
Formulated
concentration
Valid chemical test
method
CERTIFIED REFERENCE VALUE
TRACEABLE REFERENCE VALUE
WITH UNCERTAINTY ESTIMATE
PARTICIPANT LABORATORIES
1
For a pure chemical substance, the kg and the mole are related through molecular weight.
The kilogram is the SI unit of mass. It is equal to the mass of the international prototype of the
kilogram held at the Bureau Internationale des Poids et Mesures (BIPM), Paris.
3
Copy number 44 of the International Prototype Kilogram. Held at the National Measurement Institute,
Lindfield, NSW.
2
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8.2 Assignment and Verification of the Reference Values
The gas mixtures for this study were prepared gravimetrically using a Sartorius CC10000S
mass comparator. Environmental conditions were monitored during the use of the mass
comparator to allow for the correction of buoyancy. The purity of the source gases was also
included in the calculation of mixture composition and for the assignment of the concentration
of each gas component.
The samples for this study had their compositions verified analytically prior to distribution. Gas
mixtures were verified using a Bruker 456GC Natural gas analyser (configuration C). Column:
60m x 0.25 mm CP-Sil 5 CB, df = 1µm to FID, plus a Varian 3800 gas chromatograph with a
6’ x 1/8” Hayesep R column with TCD and FID in series. During the measurements, each
sample cylinder was equipped with a conditioned gas regulator. The regulators were
connected to a VICI sample selection valve and then to a common Bronkhorst mass flow
controller to give a highly consistent sample pressure and flow through the GC sample loop.
8.3 Measurement Uncertainty of the Reference Values
The gravimetric uncertainty of the gas mixtures was calculated using the principles described
in ISO 6142, 20014. The gravimetric uncertainty budget included contributions from:
•
Balance uncertainty
•
Buoyancy of cylinders
•
Expansion of cylinders
•
Tare mass uncertainty
•
Tare mass buoyancy
•
Purity of gases
•
Molar mass
An uncertainty for the verification of the gas samples was calculated from the GC
measurements using the mathematical models for single-point and two-point bracketed
calibrations. The verification uncertainty included uncertainties from the measurement of
reference standards and the sample cylinders.
The uncertainty of the reference values included components from the gravimetric preparation
of the mixtures and the uncertainty due to the verification of the mixture composition by
analysis. The combined total uncertainty for each gas component was calculated by taking
the square root of the sum of the squares of these uncertainty sources. The uncertainty of the
reference values complies with ISO Guide 34, 20097 and the future version of ISO 6142.
9 References
64
1.
ISO/IEC 17025:2005. “General requirements for the competence of testing and
calibration laboratories.” 2005, ISO, GENEVA.
2.
JCGM 100: Evaluation of measurement data - Guide to the expression of
uncertainty in measurement. 2008, JCGM.
3.
Eurachem/CITAC Guide “Quantifying uncertainty in analytical measurement.”
2000, Second Edition, http://www.eurachem.ul.pt/guides/QUAM200-1.pdf
4.
ISO 6142:2001. “Gas analysis - Preparation of calibration gas mixtures Gravimetric method.” 2001, ISO, GENEVA.
65
5.
Thompson M. and Wood. R., “International harmonized protocol for proficiency
testing of (chemical) analytical laboratories.” J. Assoc. Off. Anal. Chem., 1993,
76, 926-940.
6.
International vocabulary of metrology — Basic and general concepts and
associated terms (VIM). JCGM 200:2008
7.
ISO Guide 34: 2009. General requirements for the competence of reference
material producers, 2009 ISO, GENEVA.
8.
ISO 6976:1995. Natural gas - Calculation of calorific values, density, relative
density and Wobbe index from composition. 1995 ISO, GENEVA.
9.
AS ISO 13443-2007. Natural gas - Standard reference conditions. SAI Global
Appendix 1
List of laboratories that participated in this proficiency testing study:
BOC GASES NEW ZEALAND
VIVA ENERGY REFINING PTY LTD
PENROSE, AUCKLAND NEW ZEALAND
CORIO, VIC AUSTRALIA
CONOCO PHILLIPS.
LABORATORY
EXXONMOBIL PNG
DARWIN
LNG
PORT MORESBY. PNG
WINNELLIE NT AUSTRALIA
SANTOS MOOMBA OPERATIONS
RENEGADE GAS PTY LTD
PORT ADELAIDE
(SUPAGAS NSW AND SUPAGAS QLD)
SA AUSTRALIA
INGLEBURN NSW AUSTRALIA
CHEVRON, GORGON LABORATORY
WA AUSTRALIA
66